Why does an X represent a kiss?

Writing Christmas cards earlier, I asked my son if he wanted to put a kiss on any of the cards for his friends. He then carefully drew a big circle around his best friend's name and was confused when I explained that a kiss is normally written as an "X". He thought it meant treasure as in "X marks the spot"....

I couldn't explain why and it made me wonder: why does an X represent a kiss?

jonaj's response is probably the correct one but I like the theory that the 'x' is meant to look like pursed lips. I also read somewhere that it's based on the sound of the word and the sound of the letter i.e when you use 'x' in a word it sounds like 'kiss.'